1. Field of the Invention
The invention relates to a planographic printing plate precursor capable of direct printing with laser beams and a stack of plural planographic printing plate precursors.
2. Description of the Related Art
PS plates having an oleophilic photosensitive resin layer formed on a hydrophilic support have been widely used as planographic printing plate precursors. The PS plates have been produced commonly by subjecting the photosensitive layer of the plates to mask exposure (surface exposure) through a lith film, and then dissolving and removing the non-image regions thereof.
In recent years, digital technology, by which image information is processed, stored, and outputted electronically by computer, is becoming more and more popular. Accordingly, various newer image-output methods compatible with the digital technology have been commercialized. As a result, there is an urgent need for the “computer to plate (CTP) technology” that allows direct production of printing plates by scanning a high-directivity light such as laser beams according to digitalized image information without using the lith film, and thus for the planographic printing plate precursors that are compatible with the CTP technology.
As the planographic printing plate precursor compatible with such scanning exposure, a planographic printing plate precursor of a hydrophilic support and an oleophilic photosensitive resin layer formed thereon (hereinafter, referred to also as a photosensitive layer), the photosensitive layer containing a photosensitive compound that can generate an active species such as a radical, Bronsted acid or the like by laser exposure, has been proposed and already commercialized. A negative planographic printing plate precursor can be obtained by subjecting the planographic printing plate precursor to laser scanning according to digital information to generate an active species, utilizing this action to cause physical or chemical change to the recording layer to insolubilize the recording layer, and then subjecting the recording layer to development.
In particular, a planographic printing plate precursor composed of a hydrophilic support; and onto which a photopolymerizable photosensitive layer containing a photopolymerization initiator superior in sensitization speed, an addition-polymerizable ethylenic unsaturated compound, and a binder polymer soluble in an alkaline developing solution; and optionally an oxygen-blocking protective layer (see, for example, Japanese Patent Application Laid-Open (JP-A) No. 10-228109) is a desirable printing plate precursor with favorable printing properties, i.e., having a high degree of productivity, easy to handle in a developing process, and having excellent resolution and inking property.
On the other hand, in terms of productivity in producing photopolymerizable planographic printing plate precursors, which requires only a simple and easy developing process, shortening of time necessary for an exposure step has become important. In the exposing step, the printing plate precursors are generally supplied in the form of a stack with slip sheets interleaved between the precursors. The slip sheets serve to prevent the precursors from sticking to each other, or from getting scratches that occur when the relatively soft surface of a photosensitive layer or a protective layer formed on the photosensitive layer is scraped against the surface of an aluminum support or a backcoat layer formed on the backside of the support. Therefore, the time for removing the slip sheets has been one factor of inefficiency. Efficiency in the exposure step can thus be promoted by eliminating the need of removing slip sheets, i.e., providing a stack of precursors without slip sheets therebetween. Consequently, it has been desired to improve adhesion resistance of a precursor against another precursor and prevent scratches caused when the surface of the protective layer is scraped against the aluminum support.
The scratches mentioned here refers to those occur on the top surface side (the side with a photosensitive layer or protective layer). There are serious problems such as smudging of printing materials caused by curing of the photosensitive layer having scratches on a non-image area thereof, and blanking in images caused by scratches formed on an image area.
As the solutions for such problems as above, there have been proposed a method of providing a backcoat layer having a Tg of 20° C. or more (see, for example, JP-A No. 06-332155), a method of using a polyester resin, phenoxy resin or polyvinylacetal resin in a backcoat layer against a positive-type photosensitive layer (see, for example, JP-A No. 2005-62456), and the like. However, further improvements are still desired.
As mentioned above, there still is a demand for a planographic printing plate precursor having improved resistance to scratches formed upon scraping with each other and slipping of the plate precursors, even when stacked without interleaving slip sheets, thereby serving to promote productivity in a plate making process.